The pressure relief device (PRD) having a straight cavity filled with fusible composite alloy acts as a safety device of pressure vessels in case of fire by fusing of the alloy, thus releasing the pressurized gas to prevent the explosion of pressurized vessels or any other pressurized hardwares.
To improve the safety of pressure vessels of low thermal conductivity, the response time of the fusible alloy to fire must be as short as possible. This is especially so when a composite cylinder of poor thermal conductivity such as a plastic composite cylinder comprised of a plastic liner wrapped with filaments is used as a pressure vessel. For a typical 12 inch diameter.times.50 inch length composite cylinder, a response time of 2 minutes or less is desired to prevent the rupture of cylinder. In order to achieve the short response time, the amount of fusible alloy sensing the heat must be small and the location of PRD must be close to the outside/external fire or heat source. The purpose of the present invention is to present a new PRD design which allows the fast response time by placing the alloy-filled cavity at the outer atmospheric region and also by reducing the volume of fusible alloy, while the flow rate is not affected or independent of the alloy volume.
When the ambient temperature rises, the metallic shell of PRD (brass, bronze, steel, or stainless steel) conducts heat fast and the PRD cavity wall becomes hot first and thus the alloy skin layer becomes hotter than the alloy core region. Like an ice falling off the container, the fusible alloy slug of cylinder shape contained in the straight cavity will be pushed/expelled out of the cavity as soon as the thin alloy layer at the cavity wall begins to fuse. This projectile must be stopped to prevent any damage/harm which could occur when the gas is vented. When a venting pipe is used in a vehicle driven by the natural gas of compressed natural gas (CNG) cylinder to vent the released gas away from the heat, such pushed/expelled alloy slug must not be trapped in the venting pipe. Very often the venting pipe is bent at several points to accommodate the specific vehicle space structure and consequently such bent spots can easily trap the expelled alloy slug, thus blocking the gas flow path and delaying the gas activation/release time. When the release time is long, the cylinder explosion could occur. What is required is a device which can trap the alloy while at the same time allowing the gas to flow. This is another objective of the present invention. The composite alloy as described in detail in prior art of Ser. No. 08/047,201, filed Apr. 16, 1993, Pat. No. 5,419,357, consists of fusible alloy matrix phase and reinforcing agents such as fibers, spherical shots, particles, random aggregates, cylinders, platelets, cubes, and any other shapes. Nickel-coated steel shot is the first candidate for mass production but the nickel coating process for prolific number of shots is very expensive and time consuming. Therefore, economical reinforcing agents are required for mass production and this is yet another objective of the present invention.